1. Field of the Invention
The invention generally relates to reducing the power consumption of memory devices. More specifically, the invention relates to reducing the power consumption of memory devices due to defects in the memory device.
2. Description of the Related Art
Many modern electronic devices contain digital memory (e.g., a dynamic random access memory, DRAM). Each memory may be used to store information for a digital device. Users of such electronic devices typically desire large amounts of memory in a small package. Where the electronic devices are portable (e.g., battery powered), users may also desire electronic devices which do not consume as much power and therefore have a longer battery life. Thus, manufacturers of electronic devices typically desire small, high density memories with low power consumption.
To meet the demand for small, high density memories, memory manufacturers typically create memory chips with the smallest available features (e.g., transistors and control lines) and with densely packed memory cells. However, as the size of features in a memory chip shrinks and as the memory density of a memory chip increases, small, uncontrollable errors in the manufacturing process may cause more defective memories at a high cost to the manufacturer.
An example of a possible defect in a memory device is a short (e.g., an unintended electrical connection) between a defective wordline and a bitline of a memory device. Wordlines and bitlines are lines in a memory device which are used to access a memory cell (e.g., at the junction of the wordline and bitline) in the memory device. Where a wordline and bitline are shorted, the memory cell located at the junction of the wordline and bitline may, in some cases, not be accessible, thereby resulting in a defective wordline and defective memory device.
To reduce the number of defective memory devices resulting from defective wordlines and/or bitlines, many manufacturers create memory devices with redundant wordlines and/or bitlines. Where, for example, the manufacturer detects a defective wordline, the memory device may use a redundant wordline instead of the defective wordline. When a memory device uses a redundant wordline instead of a defective wordline, the defective wordline may be referred to as a repaired wordline.
While repairing a defective wordline may prevent a memory device from losing data, the defective wordline may still remain shorted to a bitline. In some cases, where a defective wordline is shorted to a bitline, current may flow between the defective wordline and the bitline, thereby increasing the power consumption of the memory device.
As an example, when wordlines and bitlines are not being used to access memory cells, each wordline is deactivated and the bitlines are precharged in preparation for the next sensing operation. During deactivation, each wordline voltage may be lowered to a low voltage (e.g., the low wordline voltage, VNWLL), thereby disconnecting each memory cell from the bitlines used to access the memory cell. The voltage of the bitlines when properly precharged is a voltage midway between the voltage corresponding to a bitline high logic level (VBLH) and the voltage corresponding to a bitline low logic level (VBLL). The midway voltage ((VBLH+VBLL)/2) may be referred to as the precharge voltage or bitline equalize voltage, VBLEQ.
However, where a wordline is defective (e.g., shorted to a bitline), during precharge, current may flow from the bitline being held at the precharge voltage (VBLEQ) to the defective wordline being held at the low wordline voltage (VNWLL). Where current flows between a defective wordline and a bitline, the power consumption of the memory device may increase. In some cases, because each defective wordline may be in a precharge state for an extended period of time (e.g., when the memory device is not being accessed but is in a standby state), and because each memory device may contain several defective wordlines, the power consumption of the memory device during precharging due to defective wordlines may be substantial, thereby decreasing the value of the memory device (e.g., by reducing the battery life of electronic devices which use the memory device).
Accordingly, what is needed are methods and apparatuses for reducing the power consumption of a memory device due to a defective wordline.